After the guilty-pleasure success of 2010's Piranha 3D, the quickly slapped-together trailer for this sequel looked like just as much fun. Sadly, more time and creativity was put into that teaser than the finished movie, which is a choppy, unfunny mess.

Maddy (Panabaker) is back home in Arizona from grad school, working in the water park she owns with her breast-obsessed stepdad Chet (Koechner). But after the Lake Victoria disaster, prehistoric piranhas have migrated here, drawn to the park's chlorine. After consulting with wild-haired expert Goodman (Lloyd), Maddy tries to avert disaster with the help of deputy Kyle (Zylka) and nice-guy Barry (Bush), who are rivals for her affections. But as the summer launch party nears, Chet refuses to close the park.

The screenwriters use trite plotting to distract from the fact that the characters aren't even weak stereotypes. They have no personalities at all, aside from what the cast members try to inject into the bland dialog. Maddy has a few friends whose romantic liaisons are interrupted by grisly piranha incidents, but these side-stories are so belaboured that they feel like jokes without punchlines. Even Hasselhoff looks exhausted as he lampoons his Baywatch persona as a celebrity lifeguard.

We might have gone along with this if there was a sense of structure, or if the jokes were actually funny. But the lazy writers and director never bother to hold anything together. Set pieces fall apart before they build up momentum, cameos flop because adept performers are given nothing to do or say, and the big action scenes have no logic to them at all. In fact, the film feels like it was edited together from footage shot for another movie altogether.

Nothing that happens on screen seems to have any reasoning behind it, and the piranhas themselves make no sense. They jump, but one stays in an aquarium without a lid? They swim lightning fast, but the stars easily get away? Even the cheap 3D gags are badly set up. In desperation, the filmmakers continuously cut to either something disgusting or more bouncing boobs. So besides being rampantly misogynistic, the film is no fun to watch. Even the closing-title outtakes are lame.